Bistable Nematic Liquid Crystal Research Articles

Defect Dynamics in Anomalous Latching of a Grating Aligned Bistable Nematic Liquid Crystal Device

Deliberate manipulation of topological defects is of particular interest for liquid crystal applications. For example, surface bistability occurs in the grating aligned Zenithal Bistable Device due to the stabilisation of ±½ defects at the points of high surface curvature. Conventional latching between continuous and defect states has previously been simulated satisfactorily using Q-tensor models that include the effect of weak-anchoring and flexoelectricity. However, experimental studies show that some arrangements lead to anomalous latching regimes. The Q-tensor model is used to show that such effects occur when the defects become detached from the surface and have more complex paths in the bulk of the sample.

Discontinuous Galerkin finite element methods for the Landau–de Gennes minimization problem of liquid crystals

Abstract We consider a system of second-order nonlinear elliptic partial differential equations that models the equilibrium configurations of a two-dimensional planar bistable nematic liquid crystal device. Discontinuous Galerkin (dG) finite element methods are used to approximate the solutions of this nonlinear problem with nonhomogeneous Dirichlet boundary conditions. A discrete inf–sup condition demonstrates the stability of the dG discretization of a well-posed linear problem. We then establish the existence and local uniqueness of the discrete solution of the nonlinear problem. A priori error estimates in the energy and $\mathbf{L}^2$ norms are derived and a best approximation property is demonstrated. Further, we prove the quadratic convergence of the Newton iterates along with complementary numerical experiments.

Method for Tuneable Homeotropic Anchoring at Microstructures in Liquid Crystal Devices

A simple method for vapour-phase deposition of a silane surfactant is presented, which produces tuneable homeotropic anchoring in liquid crystals. Both the zenithal anchoring energy and surface slip are measured by fitting to the latching threshold versus pulse width characteristic of a zenithal bistable nematic liquid crystal device based on a deep, submicron grating. The method is shown to give microscopic anchoring strength between 5 × 10-5 and 2 × 10-4 J/m2, with a surface slip of about 100 nm. The silanated surfaces are characterized using atomic force microscopy and X-ray photoelectron spectroscopy, which show a direct relationship between the surface coverage of silane groups and the resulting anchoring energy.

Bistable chiral nematic liquid crystal device switchable between the transmissive and reflective modes

Proposed herein is a single-panel liquid crystal device that is switchable between the transmissive and reflective modes without subpixel division. Dye-doped liquid crystals were used to eliminate the image inversion caused by a reflective polarizer. The splay and π-twist states of the bistable chiral splay nematic mode, which has two stable states, were employed for a good dark state in both the transmissive and reflective modes. Operation in the transmissive mode can be achieved by applying an in-plane field, whereas a vertical field is applied for operation in the reflective mode. It is expected that the proposed device can be used as a highly efficient transflective device for outdoor as well as indoor environments.

Spectral modulation of a bistable liquid-crystal photonic structure by the polarization effect

The light polarization has an effect on spectral properties of a multilayered photonic crystal infiltrated with a bistable chiral-tilted homeotropic nematic liquid crystal (LC) as a defect layer. By varying the direction of polarization of incident, linearly polarized light interacting with the birefringent LC, the tunability of defect modes in wavelength and amplitude and the broadening of the low-transmittance range can be realized in the transmission spectrum. The LC features two optically stable states and two voltage-sustained states. The bistability makes the device of low energy consumption. Such a hybrid can be used as not only a wavelength selector, optical shutter or multichannel switch but also a stopband-tunable device.

Soluble polyimide films as alignment layers for bistable chiral-tilted homeotropic nematic liquid crystal display applications

Soluble polyimides (PI) with and without aliphatic side chains were synthesized by a one-pot synthesis method and acted as homogeneous or homeotropic liquid crystal alignment layers. Homeotropic PI was synthesized by grafting an alkyl side chain on the backbone of homogeneous PI. The vertical alignment properties can be tuned by adjusting the grafting density and the length of the side chains. In addition, various homogeneous and homeotropic PIs were blended by a different ratio to adjust the pretilt angle of the alignment film. The long-term alignment properties and thermal resistance of the alignment films were investigated. Combining the dual frequency liquid crystal with the alignment layers which have an intermediate pretilt angle of 75°, a bistable chiral-tilted homeotropic nematic liquid crystal display device is demonstrated.

Bistable liquid crystal devices with nanoparticle-coated polyimide alignment films

Bistable hybrid-aligned nematic (HAN) liquid crystal devices (LCDs) with silica nanoparticle-coated polyimide alignment films were investigated. It was observed that the existence of the internal electric field produced from the triboelectrically charged silica nanoparticles layer and impurity ions in the LC reduced the total free energy of the HAN-LCD and stabilized the cell in the homeotropic state. The stable homeotropic state can be switched back to the HAN state by changing the ion distribution through a voltage pulse with proper polarity. The capability of controlling bistability through modification of ion density in the LC layer may have some applications, such as displaying a one-time password requiring a specific stable time.

Flexible bistable chiral nematic liquid crystal display with enhanced memory characteristic by surface treatment

AbstractWe proposed a flexible bistable chiral splay nematic liquid crystal display with the enhanced memory characteristics by the surface treatment with reactive mesogen (RM). With the polymerized RM structure on alignment layers adopted, the energy barrier between splay and π‐twisted states in the bistable chiral splay nematic liquid crystal mode is increased because of the enhancement of the azimuthal anchoring energy, and thus, the spontaneous relaxation from the π‐twisted state to the initial splay state is remarkably impeded. As a result, the memory retention time became twice as long as that of the conventional cell without the polymerized RM structure, and the stable memory characteristics were maintained against the external deformation.

Tunable bi-functional photonic device based on one-dimensional photonic crystal infiltrated with a bistable liquid-crystal layer

We investigated the optical properties of a one-dimensional photonic crystal infiltrated with a bistable chiral tilted homeotropic nematic liquid crystal as the central defect layer. By modulating the nematic director orientation with applied voltage, the electrical tunability of the defect modes was observed in the transmission spectrum. The composite not only is a general tunable device but also involves the green concept in that it can operate in two stable states at 0 V. Under the parallel-polarizer scheme, the spectral characteristics suggest a potential application for this device as an energy-efficient multichannel optical switch.

반사형 쌍안정 카이랄 스플레이 네마틱 액정표시소자

Bistable chiral splay nematic liquid crystal display (BCSN LCD) is a memory type liquid crystal display using splay and twist states as two stable states. When the cell thickness to pitch (d/p) ratio is 0.25, splay and twist states have permanent memory time. However, when the transition from twist state to splay state is caused by a fringe field, pixel regions show that the splay state is not perfect, but rather includes a contribution from the twist state. In this paper, we propose a reflective BCSN LCD using twist state in which the two stable states do not coexist. The fabricated reflective BCSN LC cell shows a high contrast ratio of over 30:1 and response times of 950 ms and 450 ms in vertical and fringe field switching, respectively. The proposed cell also shows wide viewing angle characteristics of in left- and right directions.

Dual mode operation of bistable chiral splay nematic liquid crystal cell using horizontal switching for dynamic operation

We propose a dual mode liquid crystal display (LCD), which can realize dynamic and memory operating LCD modes in a display panel. Horizontal switching and bistable chiral splay nematic LCD modes are used for the dynamic and memory operations, respectively. As the horizontal switching is applied to the dynamic operation, it is possible to use the same LC texture of splay for realizing dark states of both the dynamic and memory operations. Because, for each operation, pixel division is not required in the proposed dual mode LCD, there is no decrease in aperture ratio and resolution. In order to obtain superior display performance for both operations, a wideband half-wave configuration is employed to the proposed dual mode LCD. Due to the wideband configuration, very low light leakages at the dark state and good dispersion characteristics for the entire visible range are obtained. As a result, the proposed dual mode LCD shows a high contrast ratio of over 100:1 for both the dynamic and memory operations.

Dynamic switching behavior of bistable chiral-tilted homeotropic nematic liquid crystal displays

This investigation elucidates the transient behavior associated with switching between two stable states of a bistable chiral-tilted homeotropic nematic liquid crystal device. Experimental results reveal that the applied pulse voltages and durations not only determine the final states but are interestingly related to the relaxation time. When a suitable pulse voltage is applied, the minimum relaxation time can be obtained by adjusting the pulse duration. Analyzing the hydrodynamic behavior of the liquid crystal directors explicates the switching mechanism and relaxation processes. The principles that underlie the design of the waveforms to have the minimum relaxation time are obtained.

폴리머 격벽에 의해 화소고립된 구조의 이중주파수 쌍안정 네마틱 액정셀의 전기광학 특성

We propose a novel bistable nematic liquid crystal cell, which has a dual-frequency liquid crystal material and pixel-isolating polymer wall formed by an anisotropic phase separation of a mixture of liquid crystals and UV-curable pre-polymers. The proposed cell has two stable states of left- and right-handed -twist. The switching between the two states is achieved by using a sequential waveform of low and high frequencies. A transmissive bistable liquid crystal display is designed, which achieves high contrast ratio by using the proposed cell and optical films.

Bistability of left- and right-handed π-twist states in a pixel-isolated dual-frequency nematic liquid crystal cell

The authors propose a bistable nematic liquid crystal cell which has two stable states of left- and right-handed π-twist. The switching between the two states is achieved by using a dual-frequency liquid crystal material and sequential waveform of low and high frequencies. Each state is stabilized by a multidimensional anchoring effect of a pixel-isolating polymer wall without any chiral additives. Therefore, the elastic free energies of left- and right-handed π-twist states are almost the same. As a result, the two states show superior retention time in spite of the topological equivalence.

Crucial influence on d∕p range in bistable chiral tilted-homeotropic nematic liquid crystal cells

Wedge bistable chiral tilted-homeotropic nematic liquid crystal (BHN LC) cells with various pretilt angles are experimentally manufactured. The bistable ranges were determined experimentally and by simulation. The crucial influence of BHN LC cells on the d∕p (thickness to pitch ratio of LC cell) range in the BHN cell is determined in detail by dynamic simulation. The onset of bistability is determined by the relaxing elastic restoring force, and the relevant physical mechanism is elucidated as well.

A MATHEMATICAL STUDY OF A BISTABLE NEMATIC LIQUID CRYSTAL DEVICE

We consider a model of a bistable nematic liquid crystal device based on the Ericksen–Leslie theory. The resulting mathematical object is a parabolic PDE with nonlinear dynamic boundary conditions. We analyze well-posedness of the problem and global existence of solutions using the theory developed by Amann. Furthermore, using phase-plane methods, we give an exhaustive description of the steady state solutions and hence of the switching capabilities of the device.

Stability of Bistable Chiral-Tilted Homeotropic Nematic Liquid Crystal Displays

We investigated the effects of pretilt angle and d/ p on the stability of bistable chiral-tilted homeotropic nematic (BHN) liquid crystal (LC) devices by experiments and simulation. The stability and experimental phenomena of BHN LC devices can be explained explicitly by calculating the free energies of the devices. Among the three BHN LC device varieties, (0,2π), (-π/2,3π/2), and (π/2,5π/2), the former two varieties are truly bistable devices in the high pretilt angle regime. However, the π/2 and 5π/2 states of the (π/2,5π/2) BHN LC device will always relax to the intermediate 3π/2 twisted state. We also analyze the energy barriers between the two stable states of (0,2π) and (-π/2,3π/2) BHN LC devices.

Dynamic behaviors of dual frequency liquid crystals in bistable chiral tilted-homeotropic nematic liquid crystal cell

The switching behaviors of dual frequency liquid crystals (DFLC) in a bistable chiral tilted-homeotropic nematic (BHN) LC cell are simulated and compared with the observed results. This cell can be switched between tilted-homeotropic state and twisted state under the applied voltage as low as 5V. The reason for the low switching voltage is attributed to the dielectric anisotropy character of DFLC. The analysis also shows that the switching wave form accelerates the switching behaviors of the directors which results in faster response in BHN LC cell.

Disclination Velocity in Bistable Chiral Splay Nematic Liquid Crystal Device

We investigate how miscellaneous parameters influence the velocity of the disclination line in a bistable chiral splay nematic liquid crystal (BCSN LC) device, using a model derived from the relationship of the free energy loss rate with dissipation energy. The disclination velocity is consistently coupled with the free energy difference between two stable states, rotational viscosity, and damping factor, in which the free energy difference is the most influential. The relationships between the energy difference and the parameters of the BCSN LC device were elucidated by examining the portrait of bistable curves. The calculated velocities of the disclination motion were also confirmed by comparing them with the measured results.

Multidomain Structure for Infinite Memory Time of Bistable Chiral Splay Nematic Liquid Crystal Device

To embody an infinite memory time for the bistable chiral splay nematic liquid crystal (BCSN LC) device, we propose a multidomain structure in which a π/2 twist domain is formed around the BCSN LC device. We investigate how the multidomain structure of the BCSN LC device affects the propagation of a disclination line using a subpixel model. By analyzing the multidomain structure using the subpixel model, the free energy is minimized when the disclination line is located at the boundary between the two different domains. We also demonstrate experimentally that, by forming a π/2 twist domain around the pixel of the BCSN LC device, the propagation of the disclination line can be prevented and an infinite memory time can be achieved.